Since a metal halide lamp is able to emit light which is the closest to natural light and is excellent in color rendering properties as compared with a high-pressure sodium lamp and a mercury lamp, a metal halide lamp is suitable for use as a base light for illumination of offices and shops. In recent years, a ceramic metal halide lamp which uses a discharge tube made of translucent ceramics instead of a discharge tube made of quartz glass has come into a wide use. Mercury and metal halides are enclosed in a discharge tube of a ceramic metal halide lamp.
Generally, if a general color rendering index Ra is equal to or greater than 80 (Ra≧80) (greater than 1 B of rendering class in ISO8995), it is estimated as high color rendition and if a luminous efficiency η is proximately equal to or greater than 100 (η≧100), it is estimated as high luminous efficiency. High color rendition is incompatible to high luminous efficiency and it is difficult to accomplish both of them at the same time.
Patent Literature 1 discloses that metal halides are enclosed in a discharge tube in such a manner that an excessive content of halogen atoms relative to mercury may exist in the discharge tube in order to avoid the occurrence of a black burn phenomenon and to obtain a satisfactory light flux maintenance rate. Patent Literature 2 discloses an example of a metal halide lamp in which a luminous material containing at least one kind of cerium and praseodymium is enclosed in a discharge tube. Patent Literature 3 discloses an example of a metal halide lamp with high color rendering properties and high luminous efficiency which can be attained by enclosing metal iodides as metal halides in a discharge tube. Patent Literature 4 discloses an example of a ceramic metal halide lamp which includes power supply wires to enable the lamp to stabilize arc discharge in the initial stage of lamp lighting.